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SSI Programming Model

8.2.4 SSI Receive Shift Register (RXSR)

The SSI receive shift register (RXSR) is a 16-bit shift register that receives incoming data from the serial receive data (SRD) pin. When a continuous clock is used, data is shifted in by the selected (internal/external) bit clock when the associated (internal/external) frame sync is asserted. When a gated clock is used, data is shifted in by the selected (internal/external) gated clock. Data is assumed to be received MSB first if the SHFD bit of the SCR2 is cleared. If this bit is set, the data is received LSB first. Data is transferred to the SSI receive data (SRX) register or receive data buffer register (if the receive buffer is enabled) after 8, 10, 12, or 16 bits have been shifted in, depending on the WL[1:0] control bits.

8.2.5 SSI Receive Data Buffer Register

The SSI receive data buffer register is a 16-bit buffer register used to buffer samples received in the receive shift register. It is written by the receive shift register, whenever the receive buffer feature is enabled, by setting the receive buffer enable (RBF) bit in the SCR2. When enabled, the receive data register then receives its values from this buffer register. The DSP56824 is interrupted whenever both the SSI receive data register and SSI receive data buffer register become full, if the associated interrupt is enabled. If the receive buffer feature is not enabled, this register is bypassed and the receive shift register is automatically transferred into the SRX register.

8.2.6 SSI Receive Data (SRX) Register

The SSI receive data (SRX) register is a 16-bit read-only register. It accepts data contained in the receive data buffer register if receive buffering is enabled by setting the RBF bit in the SCR2. Otherwise, it accepts data from the receive shift register (RXSR) as it becomes full. The data read occupies the most significant portion of the SRX register. The unused bits (least significant portion) are read as zeros. The DSP56824 is interrupted whenever the SRX register becomes full (when both the SRX register and receive data buffer register are full if buffering is enabled), if the receive data full interrupt is enabled.

8.2.7 SSI Transmit and Receive Control Registers

The SSI transmit and receive control (SCRTX and SCRRX) registers are 16-bit read/write control registers used to direct the operation of the SSI. These registers control the SSI clock generator bit and frame sync rates, word length, and number of words per frame for the serial data. The SCRTX register is dedicated to the transmit section, and the SCRRX register is dedicated to the receive section—except in synchronous mode, in which the SCRTX register controls both the receive and transmit sections. DSP reset clears all SCRTX and SCRRX bits. SSI reset and STOP reset do not affect the SCRTX and SCRRX bits. The control bits are described in the following paragraphs.

Although the bit patterns of the SCRRX and SCTRX registers are the same, the contents of these two registers can be programmed differently. See Figure 8-6 on page 8-7 for the programming models of the SCRTX and SCRRX registers.

Synchronous Serial Interface

8-9

Соседние файлы в папке DSP568xx